In the 1930s, Amtrak ran underground commuter trains from a hub in Manhattan. The tunnels were abandoned when cars reduced the need for that network, and now they're some of the most famous in the world.

Between 1980 and 1996, graffiti artist Chris "Freedom" Pape painted murals on the tunnel walls -- thus the unofficial name [source: Vandalog]. But the tunnel has come to mean more than that.

Depicted in the 2000 documentary "Dark Days," the Freedom Tunnel was for years home to outsiders, people no longer able to or interested in living in the world above ground. There was a whole dark city down there, right under Manhattan, now abandoned again but still displaying the art for which it was named and physical reminders of the people who lived there.

More to explore:

Columbia University tunnels: Underground network that once hosted, among other things, the Manhattan Project [source: GoG].

Hospital X: Abandoned network of buildings dating back to the early 20th century. "Jacob's Ladder" was filmed in one of its tuberculosis wards [source: GMD].